The healthy heart produces regular, synchronized contractions. Rhythmic contractions of the heart are normally initiated by the sinoatrial (SA) node, which is a group of specialized cells located in the upper right atrium. The SA node is the normal pacemaker of the heart, typically initiating 60-100 heartbeats per minute. When the SA node is pacing the heart normally, the heart is said to be in normal sinus rhythm.
Cardiac arrhythmia occurs when the heart rhythm is irregular or if the heart rate is too slow or too fast. During an arrhythmic episode, the heart's pumping action may become impaired and blood flow to peripheral tissues may be inadequate. Cardiac arrhythmias have a number of etiological sources, including tissue damage due to myocardial infarction, infection, or degradation of the heart's ability to generate or synchronize the electrical impulses that coordinate contractions. Bradyarrhythmia occurs when the heart rhythm is too slow. This condition may be caused, for example, by impaired function of the SA node, denoted sick sinus syndrome, or by delayed propagation or blockage of the electrical impulse between the atria and ventricles. Bradyarrhythmia produces a heart rate that is too slow to maintain adequate circulation. Tachyarrhythmia occurs when the heart rate is too rapid. Tachyarrhythmia may have its origin in either the atria or the ventricles. Tachyarrhythmia occurring in the atria of the heart, for example, includes atrial fibrillation and atrial flutter. Both conditions are characterized by rapid contractions of the atria. In addition to being hemodynamically inefficient, the rapid contractions of the atria may also adversely affect the ventricular rate.
Ventricular tachyarrhythmia occurs when electrical activity arises in the ventricular myocardium at a rate more rapid than the normal sinus rhythm. Ventricular tachyarrhythmia may quickly degenerate into ventricular fibrillation. Ventricular fibrillation is a condition denoted by extremely rapid, uncoordinated electrical activity within the ventricular tissue. The rapid and erratic excitation of the ventricular tissue prevents synchronized contractions and impairs the heart's ability to effectively pump blood to the body, which is a fatal condition unless the heart is returned to sinus rhythm within a few minutes.
Implantable cardiac rhythm management (CRM) systems have been used as an effective treatment for patients with serious arrhythmias. CRM system operate by delivering relatively high energy electrical shocks to the heart to terminate tachyarrhythmia and/or by delivering relatively low energy electrical pulses to one or more heart chambers, causing the heart chambers to contract at heart rate that is hemodynamically sufficient.
Pacing therapy has also been used to improve cardiac output for patients who suffer from heart failure. Heart failure is frequently related to intraventricular and/or intraventricular conduction defects, e.g., bundle branch blocks which lead to cardiac dyssynchrony and reduced pumping action. To treat heart failure, CRM systems deliver timed pace pulses that produce more coordinated contractions of the atria and/or ventricles. The pace pulses are delivered to the heart chambers at specific intervals to achieve optimal improvement in pumping efficiency and cardiac output. Cardiac resynchronization pacing may include pacing both ventricles after a specified atrioventricular delay. The ventricular paces may be delivered simultaneously or separated by a programmable offset.